The present invention relates to devices or circuits for monitoring and/or detecting the speed and/or failure of a fan and particularly when the fan is used to cool critical electronic components of a computer.
Many of the prior fan alarm circuit designs attempted to differentiate between running current of the fan and "not running" current. However, this difference is very small when compared with the difference in either current for different sized fans. Thus, a different alarm circuit was required for each size of fan and for fans with different bearings such as sleeve or ball bearing. Additionally, the inherent motor noise pulses required special filtering, because these motor pulses invariably were greater than the difference in running versus none running current. However, using different circuitry is very expensive and requires complicated tracking and logistics to ensure that the proper alarm circuit is installed.
Examples of prior art fan and/or temperature sensors or monitors are shown in U.S. Pat. Nos. 4,479,115 (Holzhauer), 4,843,378 (Kimura), 4,977,375 (Toth), 5,115,225 (Dao et al.), 5,436,827 (Gunn et al.), 5,517,175 (Brown et al.), 5,534,854 (Bradbury et al.), and 5,574,667 (Dinh et al.). (The entire disclosures of each of these patents are hereby incorporated by references.) Many of the prior art alerting systems disclosed in these patents have numerous components resulting in high costs and large units. Since the units are large, the locations where they can be mounted are limited.
One common use for the fans is to cool hard disk drives in computer systems. To efficiently handle larger amounts of data storage, larger hard drives have been and are being developed. These drives turn faster, generating larger amounts of heat. If the cooling fan for that drive slows down too much or otherwise fails, the drives can be damaged if they are not quickly shut-off or the fans quickly replaced. The drives may not spin, errors in data may result and/or they may not acknowledge requests for information. Thus, it is even more critical in today's environment that there be a reliable indication of fan slowdown or failure so that corrective action can be quickly taken to prevent the problems discussed above. Early detection also allows corrective action to be taken before the drives are shut down to prevent or minimize loss of data in progress or other problems caused to open files.